1. Field of the Invention
This invention relates to a device for and a method of aligning two bodies, and more particularly to a device for and a method of aligning a mask and a wafer in a mask aligner used in the manufacture of semiconductor integrated circuits.
2. Description of the Prior Art
To align a mask and a wafer, there is known a system in which two or three groups of alignment marks are provided on the mask and wafer, respectively. These alignment marks are scanned by an emitted beam such as a laser beam or the like and the light energy scattered by the alignment marks is received and converted into an electrical signal stream. U.S. Pat. No. 4,167,677 teaches a system of this type. There is also known a system in which the images of alignment marks are picked up by a photosensor array or the image pickup tube of a television and the electrical signals thereof are processed to obtain an alignment signal.
The problems peculiar to the prior art will hereinafter be specifically described by using an example of the prior art.
The prior art device for aligning a mask and a wafer in the manufacture of semiconductors is constructed as shown, for example, in FIG. 1 of the accompanying drawings. The alignment marks M and W on a mask 4 held by a holder and a wafer 6 placed on a stage 5 are scanned by a laser beam L emitted from a laser light source 1, deflected by a polygon mirror 2 and a beam splitter 3, and the scattered lights from the alignment marks M and W are passed through a condenser lens 7 and photoelectrically converted and detected by a detector 8. From the output thereof, the amounts of displacement of the mask 4 and wafer 6 are calculated by a control circuit 10 having a comparator 9, and X-direction (the scanning direction of the beam or the direction of the scanning lines of the television) and Y-direction (a direction orthogonal to the X-direction) drive motors 11 and 12 and a rotational direction drive motor 13 are driven to move the stage 5, thereby accomplishing alignment.
Assume, for example, that alignment marks M as shown at (a) in FIG. 2 of the accompanying drawings are depicted on the mask 4 and alignment marks W as shown at (b) in FIG. 2 are depicted on the wafer 6 and that these marks are aligned into a positional relation as shown at (c) in FIG. 2. In the apparatus of FIG. 1, when the alignment marks M and W are scanned in the direction of the arrow by the laser beam L, a signal as shown at (b) in FIG. 3 of the accompanying drawings which results from the scattered light based on the marks M and W which are in the positional relation shown at (a) in FIG. 3 is obtained from the detector 8 of FIG. 1. This signal is pulse-shaped by the comparator 9 and the spacings T.sub.1 -T.sub.5 between the pulses are found, and from these spacings, the positional relation between the mask 4 and the wafer 6 is obtained, for thus effecting aligning. Generally, however, during the setting of the wafer 6, the marks W do not always lie between the marks M as shown at (a) in FIG. 3, but sometimes they are in the positional relation shown at (a) or (b) in FIG. 4 of the accompanying drawings and also, it is difficult to discriminate which of the obtained pulses is the signal from the mark M of the mask 4 and which of the obtained pulses is the signal from the mark W of the wafer 6. After the pulse train shown at (b) in FIG. 3 has been obtained, it is necessary to extract the features of the pulse train from the spacings between the pulses on the basis of the reference width a of the mark M of the mask 4 shown at (a) in FIG. 3 and to discriminate whether the positional relation between the mask 4 and the wafer 6 is in the condition shown at (a) in FIG. 3 or at (a) or (b) in FIG. 4. However, if, as shown at (a) in FIG. 5, the marks M and W of the mask 4 and wafer 6 overlap each other on the scanning axis d of the laser beam L, a signal is lost in the overlapping portion and the number of detected pulse signals is reduced as shown at (b) in FIG. 5. In this condition, it is difficult to find the positional relation between the mask 4 and the wafer 6 and therefore, in such a case, it is necessary to effect trial-and-error driving in which the mask and wafer are moved relative to each other by several tens of .mu.m in X-direction and also by several tens of .mu.m in Y-direction until the pulse signals are separated into six signals. Also, signals in the pulse signal detected by the detector 8, as shown in FIG. 5, the width b thereof has a time expance larger than the actual width c of the marks M, W due to the spot diameter, etc. of the laser beam L. Accordingly, where, as shown at (a) in FIG. 5, the marks M and W of the mask 4 and wafer 6 come close to each other within the signal width b if they do not overlap each other, the detection signals overlap each other and therefore, a pulse signal is lost and the trial-and-error driving becomes likewise necessary. However, if an attempt is made to make the alignment marks M and W smaller, the dimension a of the alignment marks M also becomes shorter and as a result, the marks M and W of the mask 4 and wafer 6 overlap each other or come close to each other, thereby increasing the probability with which the trial-and-error driving will need to be effected. Accordingly, if the marks M and W are made excessively small, there will arise a problem that the trial-and error driving is repeated to make aligning difficult or that much time is required for aligning.